As one of methods of welding a workpiece composed of two members, friction stir welding is known. Friction stir welding is a welding method of welding a workpiece by frictional heat generated on the surface of the workpiece by rotating a tool in a state of pressing an abutting portion of the workpiece on a shoulder surface of the tool.
Incidentally, in such friction stir welding, a tool called a bobbin tool is sometimes used. In the bobbin tool type tool, a front surface-side shoulder having one shoulder surface is disposed on the front surface side of a workpiece, a back surface-side shoulder having one shoulder surface facing the other shoulder surface is disposed on the back surface side of the workpiece, and these two shoulders are connected by a probe penetrating the workpiece. In this way, the friction stir welding of the workpiece is performed by generating frictional heat by interposing and pressing the front surface and the back surface of the workpiece between the shoulder surfaces of the two shoulders, and stirring the workpiece by the probe.
In such friction stir welding, in order to prevent the occurrence of a welding defect at a welding starting position and a welding ending position, a technique is used in which an end tab is disposed at an end portion of an abutting portion of the workpiece and the welding starting position and the welding ending position are set on the end tab.
Here, PTL 1 discloses a technique in which such an end tab is provided and friction stir welding is performed by a bobbin tool. Further, PTL 2 discloses a jig which holds an end tab (a tab plate).